1. Field of the Invention
The present disclosure relates to a method for controlling resonant power converters using power transistors, particularly for controlling induction heating systems.
Power converters contain resonant L-C networks whose voltage and current waveforms tend to be quasi sinusoidal and in phase as their frequency approach the resonance frequency.
An advantage of resonant converters is that power semiconductor switching losses are limited even though a high quantity of power is supplied to the load. Several control techniques, like zero current switching (ZCS) or zero voltage switching (ZVS), can be used to reduce power loss in resonant converters.
2. Description of the Related Art
In particular, for induction heating systems used in cooking appliances, particularly in Europe, the most used control technique is ZVS. It is based on the fact that inductive current passes through the antiparallel diode before the effective turn-on of the power transistor, thus eliminating the turn-on.
Zero voltage switching (ZVS) resonant power converters are well known in literature as well as the design criteria of all their main parameters. Among them, an important parameter is the so-called dead time that refers to the time interval between two power transistor turn-on's: it is necessary to establish a certain time interval at which both of them are off in order to avoid power transistor cross-conduction.
In order to simplify design and control of the converters, often the dead-time between two-in-series power transistors is assigned a constant value for all operating conditions, expecting this value will fulfil ZVS/ZCS in all working conditions.
If the control of the power converter applies a fixed dead time for every working condition (i.e. different loads and different requested output powers) and it doesn't adapt it taking into consideration the kind of load, however, it is possible that some configurations of load and requested output power lead to non-ZVS for the power transistors and, consequently, to a loss of efficiency for the power converter and premature power transistor wear-out.
For half-bridge series resonance power converter (a system most used in Europe for induction heating systems) the standard way for controlling the power supplied to the coil is to vary the power transistor drive frequency within a extensive range, typically comprised between 20 KHz and 100 KHz.
A typical example is the half-bridge series resonant converter used as the power system of an induction heating cooktop. The series resonant circuit of this converter consists of a capacitor, an inductor and a resistance. FIG. 1 shows a schematic draft of this kind of topology.
This topology is used to produce a high-frequency electromagnetic field that penetrates the metal of the ferromagnetic material cooking vessel and sets up a circulating loop electric current. That current flows through the resistance of the metallic pan, and generates heat. Therefore, the effective load is the cooking vessel itself. Different vessels have different electrical properties, and also the same vessel has different electrical properties at different temperatures, or when positioned slightly decentralized from the centre of the induction coil. An equivalent circuit of the FIG. 1 resonant circuit is shown in FIG. 2.
The typical switching frequency range of this type of converter is 20÷100 kHz, and the preferred control technique used in induction heating is ZVS, which will be detailed in the following description.
For avoiding cross conduction between the two power transistors that will cause permanent failure of the converter, it is established a fixed dead-time between both pulse-width modulation (PWM) power transistor driving signals. It is expected that load current will flow through the antiparallel diode of opposite power transistor just before next power transistor turn-on, during the dead-time time interval. However, due to the wide range of possible work conditions in terms of different cooking vessel loads and requested output power, it is possible that this situation might not happen always and thus the ZVS conditions are not always fulfilled. An example of this is shown in FIGS. 3a-3d. 
FIG. 3a shows three different working situations for a half-bridge series resonant converter which are detailed in FIGS. 3b-3d. In the figures are indicated the Vce collector-emitter voltage at power transistor turn-on, where Vce is equal to the difference between Vdc link and output voltage Vd (FIG. 4). FIG. 3b shows ZVS control mode. FIG. 3c shows non-ZVS power transistor control mode in which the power transistor voltage drop Vce is low. FIG. 3d shows non-ZVS power transistor control mode in which the voltage drop Vce is at maximum.
The three FIGS. 3b-3d show the coil current and coil voltage of the induction coil of the converter (Lr in the schematic diagrams of FIGS. 1 and 2), and the power transistor frequencies actuated for FIGS. 3b, 3c, and 3d are 25 KHz, 23 KHz and 22 KHz, respectively.
FIG. 3b shows almost null Vce so ZVS is fulfilled. Instead FIGS. 3c and 3d show non-ZVS occurrences.